Time-of-flight (ToF) cameras can sense distance or range to an object by emitting a pulse of modulated light signal and then measuring the time differential in the returning wave front.
A ToF camera can comprise a light source such as a bank of light emitting diodes (LED) whereby a continuously modulated harmonic light signal can be emitted. The distance or range of an object from the light source can be determined by measuring the shift in phase (or difference in time) between the emitted and the reflected photons of light. The reflected photons can be sensed by the camera by the means of charge coupled device or the like.
The phase shift (or phase delay) between the emitted and reflected photons is not measured directly. Instead a ToF camera system may adopt a pixel structure whereby the correlation between the received optical signal and an electrical reference source is performed in order to determine a measure of the phase delay.
The resulting distance (or range) map can represent the distance to objects as the relative intensity of pixels within the distance map image.
However, the distance map image can be corrupted with the effect of noise, whether it is random noise as a result of thermal noise in the charge coupled device or noise as a result of systematic errors in the measurement of the distance to the observed object. In particular the operational performance of a ToF camera system can be influenced by internal factors resulting from the operational mode of the camera and external factors caused by characteristics of the sensed scene and sensing environment.
For example, internal factors which can limit the capabilities of a ToF camera system may include the physical limitations of the sensors used such as inherent noise and resolution. Other internal factors which can limit the capability of a ToF camera system can include the power of the emitted signal, and the integration time for forming the reflected signal samples.
External factors which may limit the performance of a ToF camera system may include the angle of incidence of the illuminating light onto sensed object, the light reflectivity of colours and materials of the sensed objects, the sensing range of the ToF camera system, and the returned light signal being formed by multiple reflections.
These factors can seriously impact on the precision of distance (or range measurements and operational efficiency of a ToF camera system. In particular for low-power ToF system devices the effect of noise can limit the distance sensing capability.